The present invention relates to a device to detect optical signals with means to generate at least one reference light ray which has frequency shift and/or frequency modulation or phase shift and/or phase modulation and/or time displacement over the optical signal to be detected, with means with which the optical signal to be detected and/or the reference light ray(s) can be aligned in such a way that they can be brought into interference and with at least one detector with a demodulator by means of which amplitude modulation can be detected.
The invention further relates to a device to generate optical signals by means of modulation of optical carriers with means to generate at least one reference light ray which has frequency shift and/or frequency modulation or phase shift and/or phase modulation and/or time displacement over the optical carrier to be modulated, with means with which the optical signal to be detected and/or the reference light rays can be aligned in such a way that they can be brought into interference and with at least one coupler by means of which the resulting interference signal can be coupled out.
The invention further relates to the use of a device in accordance with the invention as an optical receiver or an optical modulator or as a spectrometer.
The optical information transfer is based on different methods which are each based on the modulation of certain properties of the optical carrier wave. When a laser is used as the light source, information can be effected by time modulation of the amplitude, the frequency, the phase or the polarization of the light source, with a modulation of the polarization only being used in special cases due to technical difficulties in the transfer in optical fibers. The essential element of such methods for optical information transfer is the optical receiver which must be capable of recognizing the relevant modulation within a very short time event at a very low intensity of the signal received.
The simplest method (direct detection) comprises the measurement of the intensity of the incident optical carrier. Accordingly, only a modulation of the intensity, i.e. amplitude modulation (amplitude shift keying, ASK) can be used as the modulation method. In addition, a very strong modulation of the frequency of the optical carrier can be detected by the receiver with the aid of one or more suitable optical filters directly as amplitude modulation, too.
The modulation forms of phase and frequency modulation require the overlapping in the receiver of the incident optical signal to be detected with a local reference light source (local oscillator, LO). Here, a difference is made between homodyne detection and heterodyne detection, with the reference light source possessing the same frequency as the optical signal to be detected in the first case and a different frequency in the second case. In the so-called quasi-heterodyne detection method, only the phase position of the reference light source is varied. The overlapping of optical signal and light of the reference light source leads to an interference signal from which, depending on the arrangement, the amplitude and phase or frequency of the signal to be detected can be derived with reference to the local reference source.
Furthermore, a difference is made between coherent (coherent detection) and non-coherent (non-coherent detection) receivers. While with coherent detection, the modulation of the interference signal is evaluated with amplitude and phase position, with non-coherent detection, only the intensity of the modulation is detected, i.e., in this case, the envelope curve of the interference signal is observed. In the case of coherent detection, the local reference source must be stabilized according to frequency and phase position and must track the incident optical carrier, while with the non-coherent detection, a control of the frequency of the local reference light source is sufficient.
The homodyne detection allows the measurement of the phase and thus of the phase modulation (phase shift keying, PSK) of the optical carrier wave; heterodyne detection also allows the detection of phase jumps in the optical carrier wave (differential phase shift keying, DPSK). The heterodyne detection is also used for the detection of a frequency modulation (frequency shift keying, FSK). In this case, the different frequencies can be detected electronically in the interference signal. Both the direct detection (ASK) and the heterodyne methods can transfer several sub-carriers modulated up in the radio wave range (sub-carrier modulation (SCM). The SCM methods are gaining in significance as the bandwidth of the feasible signal connections increases since a single optical channel can be used for several independent data streams.
The following are just some of the possibilities:                Direct detection: ASK        Heterodyne, non-coherent detection: ASK, FSK, DPSK        Heterodyne, coherent detection: ASK, FSK, PSK        Homodyne, coherent detection: ASK, PSK        
The bandwidth of the signal which can be transferred and also the technical effort grow in accordance with this list. On the receiver side, coherent detection, in particular homodyne coherent detection, means a high technical effort due to the required stabilization of the local reference light source.
To cover the range between bandwidths which can still be handled and processed electronically (<GHz) and the transfer capacity of the optical systems, several optical carriers of different wavelengths can be used which use the optical system together, but which are used individually by independent transmitters/receivers. If a sufficiently high wavelength selectivity can be achieved, this wavelength division multiplexing procedure (WDM) allows the selection of a bandwidth for the modulation of the individual optical carriers suitable for the corresponding application and technology without essentially restricting the transfer capacity of the optical system (high density wavelength division multiplexing or hd WDM). One particularly interesting application field is represented by optical multi-channel networks as the ability of the receiver to select a single channel leads to a substantial simplification of the distribution nodes in the network (tunable channel, multi-access networks or TCMA).
The physical connection between the size of an optical element and its maximum possible spectral resolution does not allow spatially small elements a spectral resolution in the GHz range. While homodyne and heterodyne detection with a local reference light source allow a very high spectral resolution, the control of the reference light source requires a high technical effort.